Help drawing and design Texas Scooter boat

Discussion in 'Powerboats' started by Aransas Flats Rat, Jan 31, 2020.

  1. Aransas Flats Rat
    Joined: Sep 2018
    Posts: 190
    Likes: 10, Points: 18
    Location: Florida

    Aransas Flats Rat Senior Member

    Here is a previous post from Yellowjacket that explained a lot of my questions on plywood core strength, thank you for a great post and for sharing your knowledge and experience ....

    A layer of glass on both sides dramatically stiffens a piece of thin plywood. Typically it takes between two and three coats of epoxy to seal a piece of plywood. For the same weight, instead of using just epoxy alone, if you use the same thickness of epoxy and use glass in it you'll generally double the stiffness and the strength. That's pretty much the tradeoff. If you're going to waterproof with epoxy you're much better off putting glass in it. Really thin (3 layer) plywood isobviously going to be much stronger in one direction, but adding a single layer of glass improves the weak direction more than it does the strong direction. If you went to more cloth (either 2layers or heavier cloth) then eventually you'll get to the point where thestrength is in the glass and the wood is acting more as a core. Here is some test data that was done with a testingmachine. As you can see in the data a small amount of glass really improves the stiffness to weight ratio. Obviously the thicker the plywood the more improvement that you get fromglassing it. Also note that increasing the thickness of the plywood is more effective than adding glass on a strength to weight ratio basis. This is because the strength and stiffness go up as a cube of the thickness. But you can get similar strength to weight from the thinner plywood that is glassed, and that glassed plywood is waterproofed (if done properly) To waterproof the thicker plywood would add more weight, and wouldn't improve the strength or stiffness. For 1/4 plywood adding 4 ounces of cloth on both sides increases the weight by 20%, but it increases strength and stiffness by 50%.

    [​IMG]

    I’m sure I’ll reference it many times throughout this build.
     
    bajansailor likes this.
  2. Yellowjacket
    Joined: May 2009
    Posts: 662
    Likes: 112, Points: 43, Legacy Rep: 447
    Location: Landlocked...

    Yellowjacket Senior Member

    A single motor on a cat is a bad idea. The reason is that in a normal planing hull the bottom of the transom sets the running height of the motor. Without the hull in front of it, the water level of the motor goes up and down and you get big drag that you wouldn't have if it was behind the hull, or if you set it higher you get ventilation all the time. If you notice a lot of pontoon boats have a small section of hull ahead of their motor for just this reason. Making two hulls and trying to tie them together is also a bad idea. If you're doing it in plywood you'll need to reinforce every place where it's attached or you're looking for a place for it to crack and fail. I also don't see an advantage to a cat, or even a tunnel, you aren't trying to go that fast and all you'll get is more draft that you don't want when you're in the flats. It may ride a bit better at higher speeds, but that's probably the biggest advantage. The transom reinforcement that you showed is good so long as the stringer is one piece. If the reinforcement and stringer is two pieces with a joint under the deck that's a crack looking to a place to start, make it in one piece and avoid the potential issues. Also, there needs to be a piece of wood that is screwed to the inside of the stringer aft of the motor board to give the motor board something to push back against. Adding planing surface aft of the motor is also a bad idea. That pushes the bow down when planing. If you have a small hp motor (like 20 hp or less) it can help you get on a plane sooner and go slow more efficiently. The cat may want that if the hulls are skinny, but if the tunnel is shallow it's going to make the boar run bow low. If you're going faster than 20 mph you don't want that. The motor should be at the back of the boat. You have a ton of beam for this length of boat. You don't need any more lift aft when you're planing. You need to think about your design. Those designs (the tunnel hull for instance) are made to work as a system. With that hull you might need more lift aft, where if you have a full flat bottom hull you don't. It all has to work together. The stringer and framing you showed works, but that is likely being done in aluminum where it's all welded together. You need a piece of 1x2 in the corners where the bottom meets the sides to increase the joint surface area and at the bottoms of the stringers. If you don't have that it's going to separate at those joints.
     
  3. Aransas Flats Rat
    Joined: Sep 2018
    Posts: 190
    Likes: 10, Points: 18
    Location: Florida

    Aransas Flats Rat Senior Member

    10-4 on that and thank you. That’s what I was wanting to here. Ok so I’ll go with the original:

    Right now it is

    16’
    82” stern
    84 mid
    60 at bow

    So you are suggesting not to use a key slot transom?
    Also no tunnel?
     
  4. Aransas Flats Rat
    Joined: Sep 2018
    Posts: 190
    Likes: 10, Points: 18
    Location: Florida

    Aransas Flats Rat Senior Member

    Yes that design was from a aluminum design and it makes perfect since that it would be a mess as wood without A ton of frames.

    The original egg created system is proven and I am confident with it.
     
    Last edited: Feb 3, 2020
  5. Aransas Flats Rat
    Joined: Sep 2018
    Posts: 190
    Likes: 10, Points: 18
    Location: Florida

    Aransas Flats Rat Senior Member

    So trying to move forward with the design lol.
    7412D74A-49D8-44A6-BC53-5FA655671F0F.jpeg
    I’ll stick with this.
    It is a pure flat bottom with a shallow tunnel, I’m not married to the tunnel so I’ll leave it as an option.

    What are your thoughts on the bow rake, how far back midship Is a good starting point for the curve to start, and what’s a good angle 10, 15 or 20 degrees?

    Thanks
     
  6. Aransas Flats Rat
    Joined: Sep 2018
    Posts: 190
    Likes: 10, Points: 18
    Location: Florida

    Aransas Flats Rat Senior Member

    I finally had time to get a strong back build for this beast to get it mocked up for patterns. Here is what I have so far. Really trying to figure out the Bow rake at this point. This way I can mock up the stern and layout the sides.

    0C3EDAE7-CB49-4CC5-BD69-5D5B3EEB82F0.jpeg
    Overhead view
    B0A3AF5B-86E9-45B2-BFFB-C8329D042830.jpeg

    Bow side view of rake,
    I’m thinking this may be to steep but would like your thoughts???
    B5524C54-2FA6-4017-8988-0CD948F30F29.jpeg
    B9B03C71-CB5D-4228-9967-BDA9A01FBB15.jpeg
    Stern view:
    Stern: 82”
    Midsection: 84”
    Bow: 60”
    the key slot is cut out but I’m thinking I will stick with the full transom/stern. Any thoughts on this?

    At this point I’m trying to get a feel for the layout and lines. Also this is just cheep 1/4 luan I’m using to make patterns.

    Please comment good or bad and I welcome all suggestions. Thanks
     
  7. Aransas Flats Rat
    Joined: Sep 2018
    Posts: 190
    Likes: 10, Points: 18
    Location: Florida

    Aransas Flats Rat Senior Member

    If this works out I’d like to document all measurements, steps and pictures as it was impossible for me to find any information on building one In stitch in glue and hopefully it might help out the next guy. I look forward to your help and comments.
     
  8. Aransas Flats Rat
    Joined: Sep 2018
    Posts: 190
    Likes: 10, Points: 18
    Location: Florida

    Aransas Flats Rat Senior Member

    Bottom section layout
    4 sheets 4x8x9mm

    I created the bow and stern curve/lines by marking the sheet in 24” increments (will post actuals measurements when confident everything lines up) then used a section of 3/4 pvc tapped on 24” marks and traced out the curve.
    Most call it lofting lol

    Cut out and used for pattern on other side. Figured this way everything matched.
     
  9. Aransas Flats Rat
    Joined: Sep 2018
    Posts: 190
    Likes: 10, Points: 18
    Location: Florida

    Aransas Flats Rat Senior Member

    The boat in the background was a complete gut job took a year to complete and I just finished it. Here is what it looked like when o opened her up.
    603402FD-61FA-4CF3-B1EA-553528E750C5.jpeg
    10C77962-E5F6-4A5A-9CE6-B0D397A6288A.jpeg
    72A6CB9C-EDF1-4BD9-903D-50604E78CD28.jpeg
    It was a mess. But here it is now.
    1485628E-3FA1-492A-B556-EB02BFAD96E8.jpeg
    6E6018BC-DF8B-4682-B5ED-CEFC396215FD.jpeg
     
    bajansailor likes this.
  10. Aransas Flats Rat
    Joined: Sep 2018
    Posts: 190
    Likes: 10, Points: 18
    Location: Florida

    Aransas Flats Rat Senior Member

    Well here she is, I got the hull she’ll mocked up.
    I like the lines and I am happy with her shape.

    Ended up 16’
    82” bottom width stern
    10” sides.
    E8483203-0164-4C7F-860E-7BC218A4C529.jpeg
    970CD567-6D9B-469A-9F13-11D1EA0A6F7E.jpeg
    A19725AF-4F24-4EEA-8F8B-E121E782565B.jpeg
     
    bajansailor likes this.
  11. Aransas Flats Rat
    Joined: Sep 2018
    Posts: 190
    Likes: 10, Points: 18
    Location: Florida

    Aransas Flats Rat Senior Member

    Well I will say this is a learning but fun experience.

    Next is to get the stringer and frame measurements laid out.

    My thought on this and this may be elementary but here is how I plan to get the layout

    Run string lines at 24” intervals P to S then run string lines stern to bow just until they touch the cross strings. This should give me two things
    1. The bottom curvature
    2. The end point at the bow

    Once I get the measurements for the stringers I will mark the two center stringers at the exact measurement and I will subtract as I go out P to S to a negative 3/8 so that the deck sits flush at the sides. This should give me a 3/8 camber on the deck as well as some additional stiffness in the deck. I hope that makes sense.

    I will do the same on the Transverse frames.

    Once I’m sure stringers and frames will fit I will disassemble and start cutting out my 3/8 for the bottom and 1/4 for the sides. And then start assembling.
     
  12. Aransas Flats Rat
    Joined: Sep 2018
    Posts: 190
    Likes: 10, Points: 18
    Location: Florida

    Aransas Flats Rat Senior Member

    Ok guys I’m needing a little help on the stringer spacing and frames (scantlings) I would really appreciate your help with this. I have Read Gerr’s Book but I’m not getting the sequence of the formulas


    Here is the information I have on the overall specs materials

    Boat will used in protected shallow waters 1-2’ top speed desired 20-30 mph
    LOA: 16’
    Bottom width: 82”
    Hull depth 12”
    Draft calculation 3”

    List of weights
    60-70 hp outboard: 190
    3 persons: 600 lbs
    Rigging: 150 lbs
    Fuel: 100 lbs
    Misc: 100
    Total: 1140 I over figured on some items to allow for variance.

    Boat materials
    Hull: Epoxy wood lam
    Bottom: 4 sheets meranti 3/8 plywood
    Sides: 1 1/2 sheet 1/4 meranti
    Transom: two sheet 3/4 meranti
    Deck: 4 sheets 3/8 meranti

    Bottom and sides will be laminated with 1708 one layer.
    Inside one layer 1708
    Deck one layer of 12 oz

    This is as far as I have gotten as I don’t know we’re to start on the Scantlings.
     
  13. Yellowjacket
    Joined: May 2009
    Posts: 662
    Likes: 112, Points: 43, Legacy Rep: 447
    Location: Landlocked...

    Yellowjacket Senior Member

    Your rib spacing is dependent on how rough the water is where you're going to be running this. If you do it too wide you'll have cracking between the ribs. I'd think that about one foot would be adequate. What that rib looks like is also an issue since that is effected by your cross beam spacing. You should look at some similar kits and go from that, that's a rough idea from my experience.
     
  14. Aransas Flats Rat
    Joined: Sep 2018
    Posts: 190
    Likes: 10, Points: 18
    Location: Florida

    Aransas Flats Rat Senior Member

    Thank you Yellowjacket,

    In the plans I purchased for reference they only show two full height stringers but the hull is designed as minimal weight and the frames are show as 36” spacing.

    So if I do:
    4 stringers out of 1/2” (two pieces 1/4 laminated offset joints) that spacing would be 20” apart.

    That would be 9 frames approximately 20” apart.



    To me that seems overkill,
    And a lot of weight once taped and glassed.

    Is there a formula say, my hull is x wide x long x deep made with x thickness and spacing should be approximately this for this mph

    Boat is going to used in calm to slight chop only.

    Thanks
     

  15. Yellowjacket
    Joined: May 2009
    Posts: 662
    Likes: 112, Points: 43, Legacy Rep: 447
    Location: Landlocked...

    Yellowjacket Senior Member

    You're thinking full height stringers and if you have a good number of lateral frames you don't need as many full height longitudinal elements. But you need something to stiffen the plywood on the bottom if the longitudinal elements are too far apart. Moreover, most of time you will put the stringers on the bottom and the frames may or may not go all the way to the bottom. If they go all the way to the bottom you need limber holes to make sure no water gets trapped by a lateral frame. What I'm saying is that if you have a vertical element at two foot intervals then you need a simple stringer (like a 1x2) between the stringers to stiffen the bottom. If you did longitudinal full height stringers two feet apart (one on each side of the motor board) then you'll need a 1x3 along the middle to tie the two pieces of ply together. Then you can do full height longitudinal elements outside that at a 2 foot or 3 foot intervals, but then add the 1x2's at one foot intervals between them.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.